Re: [RFC PATCH 1/2] vmsplice unmap gifted pages for recipient

[Dave Hansen <dave@xxxxxxxx>]

On 07/25/2013 10:21 AM, Robert Jennings wrote:
> +static void zap_buf_page(unsigned long useraddr)
> +{
> +	struct vm_area_struct *vma;
> +
> +	down_read(&current->mm->mmap_sem);
> +	vma = find_vma_intersection(current->mm, useraddr,
> +			useraddr + PAGE_SIZE);
> +	if (!IS_ERR_OR_NULL(vma))
> +		zap_page_range(vma, useraddr, PAGE_SIZE, NULL);
> +	up_read(&current->mm->mmap_sem);
> +}
> +
>  /**
>   * splice_to_pipe - fill passed data into a pipe
>   * @pipe:	pipe to fill
> @@ -212,8 +224,16 @@ ssize_t splice_to_pipe(struct pipe_inode_info *pipe,
>  			buf->len = spd->partial[page_nr].len;
>  			buf->private = spd->partial[page_nr].private;
>  			buf->ops = spd->ops;
> -			if (spd->flags & SPLICE_F_GIFT)
> +			if (spd->flags & SPLICE_F_GIFT) {
> +				unsigned long useraddr =
> +						spd->partial[page_nr].useraddr;
> +
> +				if ((spd->flags & SPLICE_F_MOVE) &&
> +				    !buf->offset && (buf->len == PAGE_SIZE))
> +					/* Can move page aligned buf */
> +					zap_buf_page(useraddr);
>  				buf->flags |= PIPE_BUF_FLAG_GIFT;
> +			}

There isn't quite enough context here, but is it going to do this
zap_buf_page() very often?  Seems a bit wasteful to do the up/down and
find_vma() every trip through the loop.
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